Method and apparatus for optically enabling a circuit component in a large scale integrated circuit

ABSTRACT

According to the invention, systems, apparatus and methods are disclosed for optically enabling a circuit component in a large scale integrated circuit. In one embodiment, the invention is a circuit comprising a light sensing device for producing a signal in response to sensing light, an optic function subcircuit, and a switch connected to the light sensing device and to the optic function subcircuit for activating the optic function subcircuit when light is sensed. The light sensing device is preferably a phototransistor and a light sensing circuit is preferably placed between the light sensing device and the switch for amplifying and conditioning the light sensing signal. The optic function subcircuit can be an optical modulator, an optical receiver or any other device that is to be operated and powered only when incident light is present. The switch can be a logic gate or a transistor switch coupled to the light sensing device and to an input to the optic function subcircuit, such as a power supply or a clock input, for alternately enabling and disabling the input to the optic function subcircuit.

FIELD OF THE INVENTION

This invention relates to optical components in large scale integratedcircuits in general, and more specifically to activating a circuitcomponent in response to the detection of light at a specified point inthe circuit.

BACKGROUND OF THE INVENTION

Optical components provide and can provide many different functions inlarge scale integrated circuitry. Optical back planes are used toconnect several circuit boards to each other. Such back planes requireoptical modems to communicate between the back plane and the chips onthe circuit card. Optical communication between chips on a single cardis under development as is optical communications using optic fiber fornetworking, internet and other communications. It has also been proposedto provide optical input/output ports on microprocessors to allow fortesting and other functions. The optical interface circuitry required tosupport these applications may be used only infrequently and may involvehigh power consumption components such as analog amplifiers, sample andhold capacitors etc. Accordingly, it is preferred that the circuitrynecessary to support the optical communications only be powered when thecircuitry is in use. This saves power and reduces heat, which isparticularly desirable in a large scale.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended claims set forth the features of the invention withparticularity. The invention, together with its advantages, may be bestunderstood from the following detailed description taken in conjunctionwith the accompanying drawings of which:

FIG. 1 is a block diagram of the primary features of the invention;

FIG. 2 is a circuit diagram of an application of the present inventionto enable an optical modulator; and

FIG. 3 is a circuit diagram of an application of the present inventionto turn on an optical receiver circuit.

DETAILED DESCRIPTION

FIG. 1 is a block diagram illustrating the basic ideas behind thepresent invention. FIG. 1 shows an electro-optic circuit 10. Thiscircuit can be a modulator/demodulator, a diagnostic circuit, or anyother type of circuit that utilizes an optical input to accomplishfunctions. The electro-optic circuit can alternatively be a receiver forreceiving an optical clock signal to drive a high accuracy or high speedcomponent. The present invention is best suited to such circuits whichare used only occasionally. For example, an optical diagnostic and testcircuit is used only for test purposes and not for normal operations. Anoptical communications circuit is used only when data is sent orreceived on the optical communications line. The optical clock signalmay be provided in addition to an electrical clock signal depending onthe performance desired. Alternatively, the invention can be applied toother kinds of circuits that are not related to optical signals in anyway except that an optical signal is used to turn the circuit on andoff.

The electro-optic circuit is enabled by an enable/power up signal 12 atan enable port 14. The enable/power up signal can act on the clock,power supply or other aspect of the circuit. In any case, theelectro-optic circuit operates and consumes power only when the signalis supplied. The electro-optic circuit is connected through its port toa light sensing circuit 16. The light sensing circuit 16 receives asense signal 18 from a photodetector 20. The photodetector can beprovided with, or as part of, an electro-optic device 22. If thephotodetector is exposed to light at the appropriate wavelength and withsufficient intensity, it generates the sense signal that is received bythe light sensing circuit which in turn enables the power or theclock/data line as appropriate.

FIG. 2 shows a circuit for implementing the present invention in CMOS(Complementary Metal Oxide Semiconductor) technology such as istypically used in microprocessors and many other digital large scaleintegrated circuit chips. It is presently preferred that the presentinvention be implemented in CMOS on a silicon p-doped substrate,however, the present invention can be applied with equal effect to othertypes of integrated circuits built from other materials includinggallium arsenide and indium phosphide. In FIG. 2, light 30 falls onto aphotodetector 32 such as an n-well guard-ring phototransistor or apn-junction photodiode or any other photodetector known in the art. Thephototransistor as a result produces a small bias current at thephotodetector node 34. This node is connected to a current mirror 36.The current mirror includes two opposing p-channel CMOS transistors 38,40 with inverted gates connected to each other and to the photodetectornode 34. One side of the mirror is connected to the photodetector nodethrough the drain of one of the transistors and the other side of themirror is connected to the drain of a long gate length, highcapacitance, slow n-channel transistor 42. The gate of the slowtransistor is also coupled to the photodetector node. The slowtransistor, when switched on by the photodetector provides the currentat its drain to an amplifier stage, in this case, two inverters 44, 46.

The slow transistor with its high internal capacitance responds onlyvery slowly to the presence of a current at the photodetector node. Thisslowness not only prevents accidental triggers from noise and shock, italso prevents accidental shut downs. If the photodetector receives anamplitude modulated light pulse and produces an amplitude modulatedcurrent at the photodetector node, then the high capacitance of the slowtransistor will smooth the signal and even out the pulsed nature of thephotodetector's output. Accordingly, the slow transistor is selected tohave a capacitance that creates a response time several times slowerthan the slowest frequency of a received modulated light signal.

The amplifier stage receives the output of the current mirror, thecurrent between the slow transistor's drain and one side of the mirror'ssource to which it is connected. This signal is amplified andconditioned for digital processing by the two inverters and connected toan input of an AND logic gate. The current mirror and the amplifierstage together make up the sensing circuit analogous to the sensingcircuit 16 shown in FIG. 1. The other input of the gate is coupled to aclock/data source 50. Alternatively, it could be coupled to a powersupply line. Accordingly, when the sensing circuit produces a highoutput, the clock/data or power supply is switched on. This signal isdirected to an amplifier 52 and from there to an electro-optic device 54which is turned on through the receipt of a clock/data signal or apower/enable signal.

The electro-optic device, as explained above can be any one of severaldifferent types of optic function subcircuits on the integrated circuit.In FIG. 2, the optic function subcircuit is preferably an opticalmodulator. The optical modulator only needs to be driven when an opticalsignal 30 is present and this signal is detected by the photodetector32. At other times, the modulator is disabled and does not consume anysignificant amount of power. Since an optical modulator is typically ahigh power device with analog amplifiers, capacitors and other highpower consumption devices, turning off the modulator can significantlyaffect the chip's overall power consumption.

In FIG. 2, the photodetector 32 is shown as separate and distinct fromthe modulator 54. It is preferred that the photodetector be provided onthe periphery of the system that provides optical reception andtransmission of the modulated optical signal for the modulator,typically another photodetector and luminescent diode. The photodetectorcan also be placed within the modulator. This approach simplifies thedetection circuitry and allows the photodetector 32 to be optimized forits basic function. In addition, the light sensing circuitry ispreferably powered continuously so that light may be detected at anytime. By keeping the light sensing circuit separate from the opticfunction circuit, the light sensing circuit can be optimized for minimumpower consumption. Alternatively the modulator's photodetector (notshown) can be used instead of the one shown in the drawings or themodulator can receive the optical signals using the photodetector shownin FIG. 2.

FIG. 3 shows an alternate application of the present invention. FIG. 3shows the same photodetector 32, responding to incident light 30,sensing circuit and amplifier stage 44, 46 and like elements areindicated with like reference numerals. In the embodiment of FIG. 3, theamplifier stage output of the sensing circuit is connected to a CMOStransistor current source 56 which supplies a stable current, when thephotodetector is activated to a CMOS transistor switch 58. The drain ofthe switch is connected to an optical receiver and amplifier 60 as thepower supply input. In FIG. 3, a second photodetector 62 is connected toa signal input of the receiver. The second photodetector produces thehigh frequency demodulated optical signal that is transmitted to theintegrated circuit. The first photodetector is used only to switch onthe receiver 60. The first photodetector detects stray or perimeterlight that is directed at the second photodetector. As discussed abovewith respect to FIG. 2, the sensing circuit is a slow response circuitdue to the slow transistor 42. Accordingly, the sensing circuit outputsignal is a low bandwidth signal. The second photodetector, on thecontrary, is optimized for high frequency, high bandwidth signalreception and propagation.

As discussed above the optical receiver and amplifier 60, a rudimentaryexample of which is shown in FIG. 3, can be provided for severaldifferent functions. These functions include interconnection withoptical communication systems for data transfer, debugging anddiagnostics. The high frequency signal may alternatively be an opticalclock that is supplied to other high speed or high stability circuitry.

Importantly, while embodiments of the present invention are describedwith reference to optical communications input/output ports formicroprocessors in CMOS VLSI technology, the method and apparatusdescribed herein are equally applicable to turning on and off othertypes of circuits in other types of integrated circuits. For example,the techniques described herein are thought to be useful in other typesof circuitry, for example Bi-CMOS, bipolar transistor circuits, silicongermanium, gallium arsenide and indium phosphide systems. The techniquesherein can also be applied, for example to portable devices that haveremovable connections to fiber optic communication links.

In the above description, for the purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. In other instances, well-knownstructures and devices are shown in block diagram form. Neitherillustration is intended to be limiting in any way. It will be apparent,to one skilled in the art that the present invention may be practicedwithout some of these specific details and that many father alterationsand modifications can be made to the particular embodiments shown above.Accordingly, references to the details of particular embodiments are notintended to limit the scope of the claims but only to illustrateparticular examples. The claims alone recite those features consideredto be necessary to the invention.

What is claimed is:
 1. A large scale integrated circuit (IC) comprising:a light sensing device to produce a sense signal in response to sensinglight; a low power light sensing circuit integrated on the IC substratecoupled to the light sensing device and maintained in an active state toamplify and condition the sense signal; an optical modulator integratedon the IC substrate; a photodetector independent of the light sensingdevice, coupled to the optical modulator to provide received opticalsignals to the modulator for demodulation; a diagnostic I/O systemintegrated on the IC substrate and coupled to the optical modulator toallow optical signals to be used to communicate diagnostic protocolswith the IC; a switch integrated on the IC substrate connected to thelight sensing circuit to receive the sense signal from the light sensingdevice and to the optical modulator to produce an enable signal toactivate the optical modulator from a minimum power disabled state to apowered enabled state when light is sensed by the light sensing device.2. The circuit of claim 1 wherein the light sensing device is aphototransistor.
 3. The circuit of claim 1 wherein the optical modulatoris coupled to an optical receiver.
 4. The circuit of claim 1 wherein thelight sensing circuit comprises a current mirror to detect the sensingsignal and an amplifier to amplify the detected sensing signal.
 5. Thecircuit of claim 1 wherein the switch comprises a logic gate coupled tothe light sensing device and to an input to the optic functionsubcircuit to alternately enable and disable an input to the opticalmodulator.
 6. The circuit of claim 1 wherein the switch is connected tocouple a power supply to the optical modulator.
 7. The circuit of claim1 wherein the switch is connected to enable a clock input to the opticalmodulator.
 8. A circuit comprising: a pn-photodetector integrated on alarge scale integrated circuit (IC); a light sensing circuit coupled tothe photodetector, integrated on the IC substrate and maintained in anactive state to amplify and condition the photodetector output signal; aswitch integrated on the IC substrate coupled to the light sensingcircuit to receive the photodetector output signal and produce anenabling signal to allow power to be supplied to an optical modulatorintegrated on the IC substrate in response to detection of a signal fromthe light sensing circuit.
 9. The circuit of claim 8 wherein the lightsensing circuit comprises a current mirror in which one side of themirror includes the photodetector and the other side of the mirrorcomprises a slow transistor, the slow transistor having a gate connectedto the output of the photodetector.
 10. The circuit of claim 8 whereinthe switch comprises a gate connected to the optical modulator and to aclock signal of the optical modulator integrated on the IC substrate sothat the clock signal is supplied to the optical modulator when thephotodetector is activated.
 11. The circuit of claim 8 wherein theswitch comprises a transistor coupled across the power supply to theoptical modulator, the transistor having a gate connected to theamplifier so that the power supply is enabled when the photodetector isactivated.